It seems that something change in this mechanics since I can confirm that spending either compendium of statistical observation or memories of a much lesser self do increase dream duration by 1.

I think it would also be a nice addition to this guide, that using high intensity dreams checks is a good way to level up your main attributes (at the cost of nightmare when failing check).

In all dream you can level up Persuasion while getting intensity and as such rewards. It may not be cost effective, but if you want to level fast as still get some rewards is nice.

When choosing the Detective as your dream subject you can grind wisdom as well as persuasion, and

your intensity rewards also give you a small amount of appalling secrets that can be use to reduce your nightmares from failed check.

As a final note you can also grind a lot of Shadow by converting your long dreams landmarks into Vitreous Almanac and trade them in set of 4 for 275 CP of Shadowy gains.

So all in all. A great way IMHO, to grind 3 of your 4 main attributes.